1. Field of the Invention
This invention relates broadly to pyranometers. More particularly, this invention relates to shadow band pyranometers for measuring diffuse solar radiation throughout the course of a day.
2. State of the Art
Pyranometers are a class of actinometers that measure the combined intensity of solar radiation. Pyranometers are used largely in research, particularly meteorological research. However, they are also used in agricultural and solar energy applications.
A shadow band pyranometer is a conventional pyranometer to which a shadow band has been attached at such an angle that the shadow band blocks out the direct solar radiation throughout the course of a day. A state of the art shadow band pyranometer (available from The Eppley Laboratory Inc., Newport, R.I.) is shown in prior art FIG. 1. The shadow band 1 prevents direct solar radiation from reaching the pyranometer 2. If used in conjunction with a second pyranometer without a shadow band, direct radiation can be calculated by finding the difference between the two pyranometer measurements.
The shadow band 1 is constructed of black anodized aluminum, weighs approximately twenty-four pounds, and uses a three inch wide circular or semi-circular band approximately twenty-five inches in diameter to shade the pyranometer 2. A platform 3 at the center supports the pyranometer 2 in a level position. The height and tilt settings of the shadow band 1 must be adjusted regularly (typically every two days) to compensate for the change in solar declination angle, due to the ongoing orbital and axial change of the Earth. Various setting screws and holes 4 are provided to make this adjustment.
Another type of shadow pyranometer, shown in prior art FIG. 2, uses “shading balls” 5 rather than a band. Such an apparatus is the “Two Axis Sun Tracker” available from Kipp & Zonen USA Inc., Bohemia, N.Y. The Sun Tracker includes a motor drive 6 which can be programmed to track the movement of the sun from east to west during the course of the day. This is tracking on the azimuth axis. The motor drive can also be programmed to track on a second axis, the zenith axis. This second axis tracking automatically repositions the shading balls 5 to compensate for the changing elevation angle of the sun throughout the day. However, the apparatus is extremely complicated and costly.